Electron image-discharge device



Och 1963 P. T. FARNSWORTH ELECTRON IMAGE-DISCHARGE DEVICE Filed Nov. 17, 1954 IN VEN TOR. PHILO I FARNSWORTH United States Patent 3,108,202 ELECTRON IMAGE-DISCHARGE DEVICE Philo T. Farnsworth, Fort Wayne, Ind., assignor to International Telephone and Telegraph Corporation Filed Nov. 17, 1954, Ser. No. 469,358

9 Claims. (Cl. 313-65) The present invention relates to an electron imagedischarge device and more particularly to an image amplifier tube.

Certain image amplifier tubes of the prior art utilize apertured storage screens upon which an electrostatic charge image may be impressed and sustained for substantial periods of time. A flood beam of electrons which permeates this charge screen is modulated in accordance with the charge pattern such that a luminescent image corresponding to the pattern will be reproduced on a luminescent phosphor anode. Such a tube is commonly characterized as a storage tube, since the electrostatic image may be held on the screen for substantial periods of time during which a visible image may be reproduced. The prior art tubes usually employ a scanning beam of electrons for impressing the electrostatic charge on the screen, and a flood beam of electrons from -a suitable cathode for transferring the charge image on the screen to the phosphor anode.

Certain of the storage screens used in these tubes are composed of sheet-like insulating material alone or :of the same material supported on a metal backing. A multiplicity of tiny apertures are provided in this sheet such that the electrode corresponds to a fine mesh metallic screen. Mesh sizes of from 100 to 1,000 apertures per linear inch are now being commonly used. In both of these electrode structures, the insulator is charged by electron bombardment, the charging characteristics of the insulator depending upon, among other things, its fixed insulating qualities. Known insulators, such as silicon monoxide, have a fixed resistance which is extremely high; whereupon the charge pattern established on the insulator does not readily flow or leak therefrom even after the lapse of a considerable period of time.

The magnitude of charge on such an insulator is primarily dependent upon the extent of electron bombardment, and once this charge is established, the qualities or characteristics of the insulator material, among other things, determines the period of storage.

The present invention primarily diliers from this fixedresistance type of storage screen in that the charge de veloped upon the screen of this invention is directly dependent upon the ability of the insulating material to substantially instantaneously change in resistance. It is therefore an object of this invention to provide an electronimage discharge device with a control electrode which is variable in resistance so as to establish a predetermined charge or potential pattern thereon.

It is another object of this invention to provide an electron-image storage or conversion device which utilizes a control screen composed of a semi-conducting material having controllable resistivity such that a charge pattern may be produced for modulating a flood beam of electrons.

It is still another object of this invention to utilize a photoconductive material on the control screen of an image amplifier tube wherein the photoconductive material provides a modulating pattern of electrical potentials which conforms to a radiation image projected thereon.

In accordance with the present invention, there is provided an electron discharge device comprising a source of flood electrons, a control screen electrode for receiving said electrons, this electrode comprising a conductive metal backing and a photoconductive material on one side "ice thereof, the photoconductive side being exposed to said electrons which serve to charge the photoconductive side to a predetermined potential when the metal backing is maintained at a given potential positive with respect to the electrons, an optical means for projecting a radiation image onto the photoconductive side, which serves to alter the resistivity of the elemental areas of the latter in conformity with the elemental pattern of the radiation image, the first-mentioned potential on said photoconductive side there-by being correspondingly altered into a pattern corresponding to said radiation image, whereby the flood electrons will be modulated conformingly as they pass through the storage screen, and a phosphor anode for converting the modulated, extended electron beam into a visible pattern.

For a better understanding of the invention, together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawings, the scope of the invention being defined by the appended claims.

In the accompanying drawings:

FIG. 1 is a sectional illustration of one embodiment of this invention; and

FIG. 2 is an enlarged fragment-a1 section of the storage screen of FIG. 1.

With reference to the drawings, a direct viewing image amplifier tube is comprised of an evacuated envelope 1 having transparent opposite end faces 2 and 3, respectively. Adjacent the face 2 and substantially coaxial therewith is mounted an electron gun, generally indicated by the reference numeral 4. This gun produces a flood beam of electrons which are projected through the tube by a suitable electron lens structure. This lens structure is comprised of a silver wall coating (or similar conductive coating) 5 and an anode sleeve 6 coaxial with and axially spaced from this coating 5. The left end of the anode sleeve 6 is covered by a fine mesh metallic collector screen 7 having a mesh size ranging from to 1,000 apertures per inch. The lens structure is comprised of the three parts 5, 6 and 7 which are so arranged as to permit the beam of electrons from the gun 4 to disperse sufliciently substantially to fill the interior of the sleeve 6 and then to collimate as they approach the collector 7. Mounted on the left side (front) of the collector screen 7, as illustrated, is the control screen or grid 8 which is shown in more specific detail in FIG. 2. This control screen 8 is flat and is supported parallel to the collector 7 such that the electrons passing through the collector 7 will be directed perpendicularly to the control screen 8.

To the left or front side of the control screen 8 is a metallized luminescent phosphor anode 9 mounted on a transparent glass plate 10.

Suitable leads extend from the various electrodes just described to the exterior of the tube enevlope 1 to facilitate connection to an external power supply, shown in diagrammatic form. The phosphor display screen 9 is connected by means of a wire 11 to a positive potential, for example, five (5) kilovolts. The control screen 8 is connected by means of a wire 12 to a variable resistor 13 which may be operated such as to vary the potential on this grid from between two (2) volts positive and five (5) volts negative with respect to the cathode of the flood gun 4. A suitable battery 14 is connected between the ends of the resistor 13 with an intermediate tap of this battery leading by means of the wire 15 to the cathode of the flood lgun 4. This wire 15 also leads to the negative terminal of the anode supply battery 16 having intermediate taps connected to the wall coating 5 and the anode sleeve 6 as shown. Also, the anode 17 of the flood gun 4 is connected to this same battery.

Measuring potentials with respect to the cathode of the flood gun 4, voltages on these various electrodes are, for example, 500 volts on the wall coating 5, 300 volts on the anode 17, and 1,000 volts on the anode 6 and collector 7. As will become apparent from the following description, these voltages may be altered in any preferred manner so as to vary the operating characteristics of the tube, so that the voltages just given are typical only and approximately these values have actually been used to achieve satisfactory operation of the tube.

With reference to FIG. 2, the control screen 8 is comprised of the usual metallic screen 18 having a mesh size in the order of, for example, 500 apertures to the linear inch. On the back side (right side as viewed in FIG. 1), a photoconductive material 19 is deposited by any suitable technique such as by evaporation. The thickness of photoconductive material will vary with the type of material used and type of tube operation desired; however, a thickness of about one micron has been found suitable for certain of the well-known materials, which include selenium and antimony sulphide. While these particular photoconductive materials are specified by way of example only, it will appear to a person skilled in the art that any photoconductive material having sufficiently high resistivity may be used instead.

In operating the tube, the metallic backing 18 of the grid 8 is supplied with one or two volts positive potential from the battery 14, and the phosphor layer 9 is made several thousand volts more positive. The collector screen 7 is more positive than the metallic backing 18. These voltages are with reference to the potential of the cathode of the flood gun 4.

In operation, electrons from th gun 4 disperse toward the left and are collimated by the lens structure '5, 6 and 7. By reason of the various positive potentials, these electrons are accelerated by this lens structure and then decelerated nearly to zero as they approach the control screen 8. If the electrons permeate this screen, they are rapidly accelerated to impinge upon the phosphor 9, causing the latter to luminesce.

The potential of the metallic screen 18 is adjusted such that if any electrons fall on the photoconductor 19, the latter will be charged negatively approximately to flood cathode potential (zero volts).

With the photoconductor surface 19 at cathode potential, the electrons of the flood beam will be reflected and collected by the collector 7. Thus no electrons will pass through the control screen 8 to cause luminescence of the phosphor 9. With the potentials of the photoconductor 19 set at approximately two (2) volts positive with respect to the cathode 4, some electrons will fall on the surface 19 while the remainder will permeate the control screen 8 and produce light from the phosphor.

From the foregoing description of operation, it will now be understood that the potential of the photoconldllCtOf surface of the control screen 8 will influence the passage of electrons from the flood beam through the control screen 8 to the phosphor anode 9.

Now considering the operation in connection with the reproduction of an image by the phosphor screen 9, the backing 18 of the control screen 8 is made slightly positive, for example, one (1) volt, with respect to the cathode of the electron gun 4. By projecting a radiation image onto the control screen 8 through the tube window 2 by means of a suitable optical lens 20, the radiation wave length chosen being proper to affect the resistivity of the photoconductor 19, the highlight regions of the image on the photoconductor will cause corresponding elemental areas of the latter to reduce substantially in resistivity. Conversely, the areas of the photoconductor 19 covered with no light or something less than the aforementioned highlight, will experience little or no change from the dark resistance, which is preselected to be relatively high.

With the radiation image so projected onto the photo conductor layer 19, electrons from the flood beam will charge the photoconductor surface areas under highlights only slightly negative with respect to the metallic backing 18 by reason of the reduced resistance thereof. Stated in other words, the potential of the elemental areas of photoconductor material 19 which are under intensely bright light will be substantially the same as the contiguous metallic backing, since the photoconductor in these areas is a relatively good conductor.

Assuming that a highlight area surrounds an aperture of the grid 8, the potential around such aperture will be substantially that of the metallic backing 18 which is positive. Electrons from the flood beam will therefore pass freely through this aperture to be accelerated toward the phosphor screen 9. The number of electrons admitted by any particular aperture will therefore depend upon the intensity of radiation falling on the immediately adjacent photoconductive material.

Considering the shadow or darker regions of the radiation image as projected onto the photoconductor 19, the latter will remain a relatively poor conductor such that electrons from the flood gun falling thereon will charge the corresponding photoconductor areas more negative toward cathode potential. The flood beam electrons approaching a grid 8 aperture which is surrounded by high resistance or non-illuminated photoconductive material 19 will therefore be reflected or turned back toward the collector screen 7. Since few or no electrons can pass through this aperture, the corresponding areas of the phosphor screen 9 will remain dark.

In areas with intermediate values of incident radiation, the resistance of the photoconductor 19 is correspondingly intermediate between the highlight and dark values, producing therefore intermediate (half-tone) values of brightness of the phosphor anode 9.

Thus it is seen that highlights, shadows, and intermediate values of radiation in the incident image will be reproduced in the electron currents which exist between the control screen 8 and the phosphor anode 9. As is true in conventional storage display tubes as explained previously, the phosphor screen 9 converts these electron currents into a luminescent image which corresponds to the incident radiation image.

Typical potentials for use on the various electrodes have been given in the foregoing. Typical sizes and spacing of the electrodes appear in the following:

Inches Spacing between phosphor 9 and control screen 8 0.2 Spacing between control screen 8 and collector 7 0.1 Diameter of anode sleeve 6 1 /2 Length of sleeve 6 1 /2 Spacing between anode 6 and wall coating -5 /2 Diameter of wall coating 5 2% Length of wall coating 5 1 While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, intended in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

i1. An electron discharge device comprising a source of electrons, an electron permeable control member, means for directing said electrons through said control member, said member having two opposite side surfaces, one surface being conductive, the other surface being a semi-conductive material in conductive contact with said one surface, said source being positioned opposite said other surface, said semiconductive material changing in resistivity in response to changing incident radiation whereby a charge pattern may be formed on said other surface which corresponds to a pattern of incident radiation, and a target electrode mounted adjacent said control member for receiving said electrons after the latter pass through said control member, the charge pattern on said other surface serving to modulate said electrons whereby said target electrode receives an electron image corresponding to said charge pattern.

2. An electron discharge device comprising a source of electrons, an electron permeable control screen, means for directing said electrons through said control screen, said screen having two surfaces, one surface being conductive and adapted to be charged to a potential which is [fixed with respect to the potential of said source, the other surface being composed of a semi-conductive material which may be uniformly charged by said electrons, said semi-conductive material changing in resistivity in response to changing incident radiation whereby a charge pattern may be formed on said other surface which corresponds to a pattern of incident radiation, and a target electrode mounted adjacent said control screen for receiving said electrons after the latter pass through said control screen, the charge pattern on said other surface serving to modulate said electrons whereby said target electrode receives an electron image corresponding to said charge pattern.

3. An electron discharge device comprising a source ofv electrons, an electron permeable control member, means for directing said electrons through said control member, said member being flat and having opposite surfaces of different material, the material for one surface being conductive and the material for the other surface being photo-conductive, both materials being in conductive contact, said other surface being directly exposed to said source whereby said electrons may produce a potential thereon of a value different from that of said one surface, said other surface assuming a resistance pattern in response to a pattern of incident radiation, and a target electrode mounted adjacent said control member for receiving said electrons after the latter pass through said control member.

4. An electron discharge device comprising a source of electrons, an electron permeable control member, means for directing said electrons through said control member, said member being flat and having opposite surfaces of different material, the material for one surface being conductive and the material for the other surface being photo-conductive, both materials being in conductive contact, said other surface being directly exposed to said source whereby said electrons may produce a potential thereon of a value different from that of said one surface, said other surface assuming a conductivity pattern corresponding to an incident radiant energy image whereby the aforesaid potential will be correspondingly changed into a potential pattern, and a target electrode mounted adjacent said control member for receiving said electrons after the latter pass through said control member, the potential pattern on said other surface serving to modulate said electrons whereby said target electrode receives an electron image corresponding to said potential pattern.

5. An electron discharge device comprising a source of electrons, an electron permeable control member, means for directing said electrons through said control member, said member being flat and having opposite surfaces of different material, the material for one surface being conductive and the material for the other surface being photoconductive, both materials being in conductive contact, said other surface being directly exposed to said source whereby said electrons may produce a potential \thereon of a value different from that of said one surface, said other surface assuming a conductivity pattern corresponding to an incident radiant energy image whereby the aforesaid potential will be correspondingly changed into a potential pattern, and a target electrode mounted adjacent said control member for receiving said electrons after the latter pass through said control member, the potential pattern on said other surface serving to modulate said electrons whereby said target electrode receives an electron image corresponding to said potential pattern.

6. An electron discharge device comprising a source of flood electrons, a control screen having a plurality of apertures for receiving said electrons, electron optical means for directing said electrons substantially uniformly over said control screen, said control screen being composed of a conductive metal backing and a photoconductive material on one side thereof and in conductive contact therewith, the photoconductive side being exposed to said source of electrons, said electrons serving to charge said photoconductive side to a predetermined potential when said metal backing is maintained at a given potential which is positive with respect to the source of said electrons, a phosphor display screen mounted adjacent aud substantially parallel to the metal-backing side of said control screen whereby electrons from said source passing through said apertures will impinge upon said display screen to cause luminescence thereof, and means for projecting a radiation image onto said photoconductive side which serves to alter the resistivity of the elemental areas of the latter in conformity with the elemental pattern of said image, the aforesaid charge on said photoconductive side thereby being correspondingly altered into a pattern corresponding to said image whereby said electrons will be modulated as they pass through said apertures to produce a visible image on said display screen.

7. An electron discharge device comprising a source of flood electrons, a control screen having a plurality of apertures for receiving said electrons, electron optical means for collimating and directing said electrons toward said screen along a path substantially perpendicular to the latter, said electron optical means including a fine mesh conductive screen positioned parallel to and adjacent said control screen for providing a parallel equipotential surface, said control screen comprising a conductive metal backing and a photoconductive material on one side thereof and in conductive contact therewith, the photoconductive side being exposed to said source of electrons, said electrons serving to charge said photoconductive side to a predetermined potential when said metal backing is maintained at a given potential which is positive with respect to the source of said electrons, a phosphor display screen mounted adjacent and substantially parallel to the metal-backing side of said control screen whereby electrons from said source passing through said apertures will impinge upon said display screen to cause luminescence thereof, and means for projecting a radiation image onto said photoconductive side which serves to alter the resistivity of the elemental areas of the latter in conformity with the elemental pattern of said image, the aforesaid potential on said photoconductive side thereby being correspondingly altered into a pattern corresponding to said image whereby said electrons will be modulated as they pass through said apertures to produce a visible image on said display screen.

8. An electron discharge device comprising an electron flood gun, a control screen having a plurality of apertures, means directing the flood electron beam from said flood gun toward said control screen along a path substantially perpendicular to the latter, means expanding the cross-sectional area of said electron beam to a size which substantially covers the side of said control screen which faces said flood gun, said control screen comprising a fine mesh metallic screen having on only the surface facing the flood gun a layer of photoconductive material, said photoconductive material being in conductive contact with said metallic screen, said flood beam serving to charge said photoconductive material to a predetermined potential when said backing is maintained at a given potential which is positive with respect to said flood gun electrons, and a phosphor display screen mounted adjacent and substantially parallel to the side of said control screen opposite said flood gun whereby electrons from said flood beam passing through said apertures will impinge said J phosphor screen and cause luminescence thereof, said photoconductive material being exposed to receive a radiation image which alters the resistivity thereof in accordance with the pattern of said image.

9. An electron discharge device comprising an evacuated envelope having an imaginary straight-line axis, an electron flood gun positioned on said axis, a planar con- .trol screen perpendicular to said axis and having a plurality of apertures, said control screen being spaced from said flood gun and being intersected at substantially its center by said axis, electron optical means directing and expanding the cross-sectional area of the flood beam symmetrically about said axis toward said control screen, said beam substantially covering the surface of said screen which is exposed to said flood gun, said control screen comprising a fine mesh metallic screen having on only the surface facing the flood gun a layer of photoconductive material, said photoconductive material being in conductive contact with said metallic screen, said flood beam serving to charge said photoconductive material to a predetermined potential when said backing is maintained at a given potential which is positive with respect to said flood gun electrons, and a phosphor display screen mounted adjacent and substantially parallel to the side of said control screen opposite said flood gun whereby electrons from said flood beam passing through said apertures will impinge said phosphor screen and cause luminescence thereof, said envelope having a Window symmetrically arranged about and normal to said axis, said window being positioned on the photoconductive side of said control screen and being transparent to radiation which is desired to be projected onto said control screen.

References Cited in the file of this patent UNITED STATES PATENTS 2,277,246 McGee Mar. 24, 1942 2,322,361 Iams June 22, 1943 

1. AN ELECTRON DISCHARGE DEVICE COMPRISING A SOURCE OF ELECTRONS, AN ELECTRON PERMEABLE CONTROL MEMBER, MEANS FOR DIRECTING SAID ELECTRONS THROUGH SAID CONTROL MEMBER, SAID MEMBER HAVING TWO OPPOSITE SIDE SURFACES, ONE SURFACE BEING CONDUCTIVE, THE OTHER SURFACE BEING A SEMI-CONDUCTIVE MATERIAL IN CONDUCTIVE CONTACT WITH SAID ONE SURFACE, SAID SOURCE BEING POSITIONED OPPOSITE SAID OTHER SURFACE, SAID SEMICONDUCTIVE MATERIAL CHANGING IN RESISTIVITY IN RESPONSE TO CHANGING INCIDENT RADIATION WHEREBY A CHARGE PATTERN MAY BE FORMED ON SAID OTHER SURFACE WHICH CORRESPONDS TO A PATTERN OF INCIDENT RADIATION, AND A TARGET ELECTRODE MOUNTED ADJACENT SAID CONTROL MEMBER FOR RECEIVING SAID ELECTRONS AFTER THE LATTER PASS THROUGH SAID CONTROL MEMBER, THE CHARGE PATTERN ON SAID OTHER SURFACE SERVING TO MODULATE SAID ELECTRONS WHEREBY SAID TARGET ELECTRODE RECEIVES AN ELECTRON IMAGE CORRESPONDING TO SAID CHARGE PATTERN. 